The present invention relates generally to optical measurements and use of diode array structures in spectrophotometers and other optical systems.
Generally self-scanned photo diode array structures operate as solid state image sensors utilizing reversed bias p-n junction photo-diodes in an integrating or storage mode. By monitoring the charge removed periodically to re-establish the initial voltage condition of a p-n junction element in the structure, one may obtain a signal proportional to the incident illumination. (See Tsang et al., "Evaluation of the Solid State Image Sensor" J. Imaging. Sci, V.29, Number 1, January/February 1985,. Examples of image sensors are p-n junction diodes as used in self-scanned photo-diode arrays, or an MOS induced junction as used in charge injection arrays (CID), and charge coupled arrays (CCD).
One application of charge storage photo-diode structures is in spectrophotometers and similar optical systems. Spectrophotometers are optical instruments that measure the amount of light transmitted or reflected by a sample as a function of the wavelength of the light See, e.g. U.S. Pat. No. 4,076,421 issued to Kishner. Pulsed xenon light enters a polychrometer via an entrance slit after reflecting off a sample. This light impinges on a grating which disperses the light and focuses it upon a linear array charge storage photodiode structure. The grating separates the incident light into its component wavelengths by deflecting each wavelength by a unique angle. The result is an image of the spectral components in the plane of the charge storage photo-diode structure. Therefore, each element of the charge storage photo-diode structure converts the photonic signal corresponding to the incident wavelength into a corresponding electrical signal.
A typical design choice for selecting a charge storage photo-diode structure in a spectrophotometer is a charge coupled device (CCD). The application of CCD technology to image sensing is described in Tseng, "Evolution of the Solid-State Image Sensor", J. Imaging Sci., V. 29, No. 1, p. 14-20 (1985). CCD's, however, have two drawbacks in a spectrophotometer intended for efficient use with a limited power supply, such as a battery pack. These two drawbacks are photo site transfer loss (PSTL) and image lag (IL).
Photo site transfer loss (PSTL), as described in "Application Note: photosite transfer loss," CCD sensors, systems and developmental technology, (1989 Fairchild Weston CCD Imaging Databook), is a phenomenon which degrades the transfer of charge packets from the photo sites into the CCD shift register(s). The effect of PSTL is observed by using a CCD which is not illuminated for a substantial period of time. A light source is then switched on the array for few successive integration periods. Assuming that the output of CCD should be V.sub.signal, then for 0&gt;PSTL&lt;100% of V.sub.signal, the .sub.output signal resulting from the first illuminated integration time will be attenuated, due to the PSTL of the CCD. The output of the CCD for the following integration times is V.sub.signal.
An interrelated phenomenon to PSTL is image lag (IL) which as described in the reference on "Image Lag" Toshiba CCD Image Sensor Databook (3rd Edition) Toshiba Corporation, occurs when the signal charge in the area under the storage electrode of the photo-diode is not completely shifted to the transfer region. Instead, part of the signal is added to the following signal. For example, when the charge coupled device (CCD) is used in a copying machine and the first copy to be made is of a white image and the second copy to be made is of a black image, the copy of the black image will come out gray instead of black because not all the signal which was generated by the white image was transferred.
Due to the PSTL and Image Lag phenomena inherent in CCD's, accurate color analysis by spectrophotometer becomes extremely difficult or inefficient. For example, readings taken after the first illuminated scan following a substantial unilluminated period are attenuated by PSTL, and the readings taken after the first non-illuminated scan indicate the presence of some decaying prior signal. In order to measure a dark object after an illuminated scan, one should wait for a certain period of time so that the amount of Image Lag has decreased to avoid false readings.
PTSL in some circumstances can be somewhat reduced by adding optical bias light or "fat zero" to the incident illumination. This bias light, however, cannot eliminate the Image Lag. As described in Toshiba CCD Image Sensor Data Book on page 24, the amount of Image Lag is influenced by the opening time of the shift gate. Therefore in applications requiring very short gating intervals Image Lag remains a problem.
The problems of Image Lag and PSTL become even more important in applications requiring a high degree of accuracy. One such application is color analysis by spectrophotometer. Many industrial applications require matching, sorting or formulating colors accurately with consistent repeatability.
With a portable spectrophotometer the problem is more acute since it must be operated from a battery for long periods of time without recharging or replacing the battery. In such applications, ideally only one illuminated scan should be taken from the color sample in order to minimize the energy used by a light source (e.g., pulsed xenon source). In addition, the light source must illuminate the sample for an extremely short duration to prevent heating of sensitive samples. Another application requiring a short duration scan is when the sample is moving such that it is available for measurement only for a short period of time. The necessity of such short duration pulses further aggravates the PSTL and Image Lag problems. As the effects of PSTL and Image Lag cannot be easily eliminated, accurate measurements by CCD's cannot be readily accomplished.